Embodiments of this invention relate generally to systems and methods for deposition of thin films and more particularly to systems and method for depositing annular (or “circular”) wedged coatings (referred to in general as “AWC”s) for circularly variable filters (CVF). A CVF is an optical element of circular shape which includes a circular substrate coated with layers of dielectric materials to produce a multilayer interference coating which has a variable thickness around the circle (i.e. an AWC). The substrate is typically a solid disk made of a semiconductor (germanium, silicon, zinc selenide, zinc sulfide, etc.), sapphire, glass or quartz. A typical CVF is shown diagrammatically in FIG. 1. Its thickness varies from a small value at 102 to a large value at 104 on a substrate 106. A CVF can serve as a narrow band-pass filter, such that a central transmitted wavelength within a predefined wavelength range varies linearly around the substrate circumference. AWCs and CVFs are exemplarily described in A. Thelen, “Circularly Wedged Optical Coatings, I. Theory”, Applied Optics, Vol. 4, No. 8 (1965), pp. 977-981 and in J. H. Apfel, “Circularly Wedged Optical Coatings, II. Experimental”, Applied Optics, Vol. 4, No. 8 (1965), pp. 983-985. Such filters have been used for many years in spectrometers and spectroradiometers.
The deposition of AWCs is normally performed in vacuum chambers using known thin film deposition techniques (e.g. evaporation or sputtering). Some AWC/CVF coating tools use “planetary systems” in which a plurality of substrates (“planets”) are mounted on a planet carrier which rotates in a first rotation (or simply “rotation”) around a central (first) axis. Two masks are positioned between each substrate and dielectric material coating targets or “sources”. Each substrate and its respective masks may be rotated in a second rotation around a second axis which is displaced radially from and parallel to the first axis. The first (planet carrier) rotation is needed to insure good control of the thickness function itself, i.e. to cancel out effects of non-uniform deposition rates in different parts of a coating chamber. The second rotation (which includes a relative rotation between masks and substrate) is needed to achieve the variable layer thickness of the coating around the substrate circumference. In most planetary coating tools, the mechanical setups are such that the various rotations are imparted by a single motor using fixed shape cams or fixed gears. This limits the rotation speeds of the various parts of the system to fixed ratios or time profiles. Consequently, a tool used for the production of a single AWC/CVF, or for the simultaneous production of a plurality of AWC/CVFs on different substrates is suitable for only one type of AWC/CVF. If a different type of AWC/CVF has to be produced, a mechanical change has to be introduced in the tool or a different tool must be used. As used herein with reference to types of coatings, “different” refers to different laws of variation of thickness vs. position on the circumference of the substrate, described by different functions. Such functions may include linear functions with different slopes, a non-linear function such as logarithmic, sinusoidal or power function, or any other analytical or tabulated function.
In view of the disadvantages of existing planetary AWC/CVF coating tools, there is a need for and it would be advantageous to have a coating tool which will render the production of different types of AWC/CVF structures less expensive and less time consuming It would be desirable to avoid the need to build different coating tools and the need to dismantle and re-mount a coating tool in a coating chamber in order to produce a new type of AWC/CVF structures, or when testing of coating performance points to unsatisfactory tool design or production.
No known planetary coating tool or system includes these features, and none has the capability to continuously yield high quality, thin film AWC/CVF structures as is possible with the apparatus of the present invention. While prior art patents and known commercial devices may disclose or suggest features analogous to some of the features disclosed herein, all are simply lacking in one regard or another. Nothing in these prior art patents and known commercial devices suggests the present inventive combination of component elements arranged and configured as disclosed and claimed herein. Prior devices simply do not provide the benefits attendant with embodiments of the invention disclosed herein.